The invention relates to fuel for a nuclear reactor and more particularly is directed to generally cylindrical axially alignable nuclear fuel pellets of the type stacked in end-to-end relationship within a tubular nuclear fuel cladding.
Nuclear reactors are described, for example, in "Nuclear Power Engineering" by M. M. El-Wakil published by the McGraw-Hill Book Co., Inc. 1963. Nuclear fuel elements are shown, for example, in U.S. Pat. No. 3,466,226 to J. L. Lass. The patent to Lass shows a nuclear fuel element comprised of an elongated tubular cladding containing a fissionable material such as uranium and/or plutonium dioxide (UO.sub.2, PuO.sub.2) sealed therein. In the present invention this fuel material is in the form of fuel pellets which are stacked in end-to-end relationship within the cladding. These fuel pellets are pressed from a powder and sintered so that they have a ceramic-like consistency before they are stacked and sealed within the tubular cladding.
Experience has shown that fuel cladding failures or perforations tend to occur at pellet interface locations due to the mechanical interaction (pellet-clad interaction) of the pellet and the cladding. Experience indicates that one contribution to such failures is the axial misalignment of adjacent fuel pellets of the stack. Axial misalignment as used herein is intended to include fuel pellets with centerlines that are displaced and/or tilted with respect to the centerline of the tubular cladding.
Axially misaligned fuel pellets may initially be due to a stochastic or random distribution of the fuel pellets within the cladding during the assembly of the fuel element. This stochastic distribution occurs because dimensional and assembly tolerances allow fuel pellets to be stacked in axially misaligned positions. Manufacturing tolerances prevent the end faces of the pellets from being perpendicular to the center axis of the pellets. Thus, as the pellets are stacked in the cladding each pellet is tilted by the deviation of its lower end face, and the cumulative deviations of the end faces of the pellets below. In most nuclear reactor fuel elements a large fraction of the fuel pellets will be tilted by the maximum amount allowed by an annular gap normally provided between the fuel pellets and the cladding. This stochastic distribution is further aggravated by the cumulative effects of fuel rod vibration, and the swelling and cracking of the pellets during irradiation.
During irradiation the fuel pellets may become locked in axially misaligned positions due to the combined effects of axial friction, (resulting from pellet swelling and from larger coefficient of thermal expansion and higher average temperature of the fuel pellets relative to the cladding) and transverse friction at the then compressively loaded pellet interfaces. Once the pellets become locked in misaligned positions, subsequent radial expansion of the pellets at higher power levels causes pellet-clad interaction which induces cladding failure. Perforations in the cladding may occur due to overstressing of the cladding or due to an increase in stress corrosion cracking of the cladding.
Cladding failures in fuel elements having misaligned fuel pellets occur at significantly lower power levels than in nuclear fuel elements where the pellets remain axially aligned. Thus, pellet-clad interaction due to axially misaligned pellets significantly lowers the absolute power level at which a nuclear fuel element may be operated without cladding failure.
Pellet-clad interaction has also forced the operators of nuclear power plants to substantially reduce the rates at which load following power changes may be made. This is because the larger coefficient of thermal expansion and the higher average temperature of the pellet relative to the cladding, the swelling of the pellet due to fission product generation, and the relatively little time for plastic deformation to occur substantially increase pellet-clad interaction during a sharp power increase. Making power changes very slowly partially alleviates this problem by allowing the pellet and cladding to plastically deform, effectively reducing cladding loads. However, these restrictions on the rates at which load following power changes may be made are particularly onerous to the operator of a nuclear power plant that must meet widely varying demands.
The prior art reveals an arrangement for attempting to align nuclear fuel pellets during the assembly of a nuclear fuel rod. See Canadian Pat. No. 725,277 to Hauser et al. However, that arrangement increases the resistance to plastic deformation at the pellet interfaces by providing pellets having interlocking male and female ends. Typically in the prior art arrangement alternate ends of the fuel pellet are provided with a projection and a projection-receiving opening, the projection-receiving opening having a depth greater than or equal to the height of the projection. When a plurality of such fuel pellets are stacked in end-to-end relationship, projections on the ends of the fuel pellets interlock with receiving openings on adjacent fuel pellets to lock the fuel pellets in position.
In the prior art interlocking pellet arrangements the fuel pellets contact each other over an annular area surrounding the projections and projection-receiving openings providing a large pellet contact area and a low mean temperature at the pellet interfaces. This large pellet contact area and low mean temperature results in low stresses in the pellets at the pellet interfaces and little plastic deformation of the pellets to reduce cladding stresses. Low stresses at the pellet interfaces result in higher cladding stresses and deformation of the cladding rather than deformation of the pellets. Prior art pellet designs having flat or "dished" ends suffer from the same disadvantages. (Dished pellets are formed with concave ends as shown, for example, by U.S. Pat. No. 3,365,371.) They have large pellet contact areas and low mean temperatures at the pellet interfaces resulting in deformation of the cladding rather than deformation of the pellets.
Another problem with the prior art interlocking pellet arrangement is the prohibitive expense of manufacture of the projection and projection receiving openings with manufacturing tolerances smaller than .+-.3 mils, and these errors have a cumulative effect on pellet misalignment. In most boiling water reactors the pellets and the cladding are separated by an annular gap of approximately 10 mils so it is still possible to get maximum tilting of the pellets with the prior art interlocking pellet arrangement.
It is the principal object of the present invention to provide a nuclear fuel pellet for use in a stacked column of pellets in a fuel element and having a shape that increases the plastic deformation of the pellet at the pellet interfaces so that mechanical interaction between the pellet and the cladding will create alignment forces great enough to deform the pellets and realign them without seriously loading the cladding.
Another object of the present invention is to provide an axially alignable fuel pellet that reduces pellet-clad interaction.
It is another object of the present invention to provide a nuclear fuel pellet which will improve the operation of nuclear power plants by increasing the rate at which load-following power changes may be made without overstressing the fuel cladding due to pellet-clad interaction.
Another object of the present invention is to reduce the stochastic distribution of pellets during the assembly of a nuclear fuel rod.
Another object of the present invention is to reduce the amount of stress corrosion cracking in the cladding of a nuclear fuel rod.
Another object of the present invention is to provide a nuclear fuel element capable of higher absolute power levels without cladding failure.